Memory devices are typically provided as internal, semiconductor, integrated circuit devices in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory has developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage (Vt) of the memory cells, through programming (which is often referred to as writing) of charge storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data state (e.g., data value) of each memory cell. Common uses for flash memory and other non-volatile memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, mobile telephones, and removable memory modules, and the uses for non-volatile memory continue to expand.
A NAND flash memory is a common type of flash memory device, so called for the logical form in which the basic memory cell configuration is arranged. Typically, the array of memory cells for NAND flash memory is arranged such that the control gate of each memory cell of a row of the array is connected together to form an access line, such as a word line. Columns of the array include strings (often termed NAND strings) of memory cells connected together in series between a pair of select gates, e.g., a source select transistor and a drain select transistor. Each source select transistor may be connected to a source, while each drain select transistor may be connected to a data line, such as column bit line. Variations using more than one select gate between a string of memory cells and the source, and/or between the string of memory cells and the data line, are known.
Programming memory typically utilizes an iterative process of applying a programming pulse to a memory cell and verifying if that memory cell has reached its desired data state in response to that programming pulse, and repeating that iterative process until that memory cell passes the verification. Once a memory cell passes the verification, it may be inhibited from further programming. The iterative process can be repeated with changing (e.g., increasing) voltage levels of the programming pulse until each memory cell selected for the programming operation has reached its respective desired data state, or some failure is declared, e.g., reaching a maximum number of allowed programming pulses during the programming operation.
A general trend in semiconductor memory fabrication is to increase memory density. This might be accomplished by decreasing feature sizes and/or utilizing three-dimensional array structures to increase the number of memory cells formed in a given area of a semiconductor wafer. However, such techniques can increase the RC (resistive-capacitive) time constant of conductive structures (e.g., access lines) used to access the memory cells, due to increased resistance levels of the structures themselves, and due to increased capacitive coupling of neighboring structures. This, in turn, can increase the time necessary to bring such an access line up to a desired voltage level for an access operation, such as a programming operation.